Recently, in order to process a massive amount of information such as images, enhancement in information density has been sought for in recording and reproduction of an optical information recording medium. In view of this, super-resolution techniques have been proposed, which record and reproduce information by use of recording marks, or pre-pits that are formed by concave portions and/or protrusions, each of which lengths of the marks or the pre-pits are shorter than a resolution limit of an optical system in a reproducing device. In the description, an optical information recording medium which is reproducible by the super-resolution technique is referred to as a “super-resolution medium” or a “super-resolution optical information recording medium”, whereas an optical information recording medium which cannot utilize the super-resolution technique, that is, an optical information recording medium which records information by use of recording marks or pre-pits, each of which have a length longer than a resolution limit of an optical system in a reproducing device, is referred to as a “regular medium” or a “regular optical information recording medium”. The resolution limit of an optical system is determined based on a wavelength of a reproduction laser and a numerical aperture of an optical system.
Optical information recording mediums disclosed in Patent Documents 1 through 3 are examples of the super-resolution medium.
A rewritable super-resolution medium described in Patent Document 1 provides a recording layer and a reproduction layer. The recording layer has information recorded thereon in a perpendicular magnetizing direction, and the reproduction layer is provided on the recording layer. Reproduction of the information is carried out by having reproduction laser light irradiated to the reproduction layer. Irradiation of the reproduction laser light to the reproduction layer causes a laser spot to generate. The laser spot thus generated has a light intensity distribution, whereby a temperature distribution is induced in the laser spot. A magnetic field of the recording layer is transferred to the reproduction layer, to just parts of the laser spot having a high temperature. This allows reproduction of a signal having a shorter marking length than the resolution limit of the optical system.
In a super-resolution medium described in Patent Document 2, a thermochromic pigment layer is provided as a mask layer on a surface of a reflection layer on which reproduction light is incident thereon. The thermochromic pigment layer changes in optical characteristic such as transmittance, depending on temperature. The mask layer is a layer which causes a super-resolution phenomenon, such as reducing a size of a laser spot in a pseudo manner. Distribution of transmittance is generated due to a temperature distribution induced by a light intensity distribution in the laser spot on the reproduction layer, near a surface of the reproduction layer on which the reproduction light is incident thereon. For example, when a material which improves in transmittance along with an increase in temperature is used as the reproduction layer, transmittance is improved in just parts having a high temperature, thereby causing the laser spot generated on the surface of the reflection layer to be reduced in a pseudo manner. This allows reproduction of a signal having a shorter marking length than the resolution limit of the optical system. The technique disclosed in Patent Document 2 is applicable not only to a rewritable optical information recording medium, but also to an optical information recording medium only for reproduction.
Furthermore, in a super-resolution medium described in Patent Document 3, information is recorded on a substrate by use of concave portions and/or protrusions. The substrate has a film layer, called a functional layer, which is made of a thin metal film or the like, provided thereon. Although substantially none of the theory of this super-resolution medium is currently known, it is known that signals which have a shorter marking length than the resolution limit of the optical system are reproducible due to a temperature change in the functional layer.
[Patent Document 1]
    Japanese Unexamined Patent Publication, Tokukaihei, No. 8-180486 (published Jul. 12, 1996)[Patent Document 2]    Japanese Unexamined Patent Publication, Tokukai, No. 2001-35012 (published Feb. 9, 2001)[Patent Document 3]    Japanese Unexamined Patent Publication, Tokukai, No. 2001-250274 (published Sep. 14, 2001)
The following description explains a conventional regular medium 61 and a conceivable super-resolution medium 71, with reference to FIGS. 14 through 18.
FIG. 15 illustrates an outer appearance of the conventional regular medium 61. The regular medium 61 includes a data area 62, and a medium information area 63. The data area 62 is to be recorded with information to be used by a user. The medium information area 63 is recorded with information concerning the regular medium 61. The data area 62 and the medium information area 63 are arranged so that the medium information area 63 is provided at an innermost circumferential section and an outermost circumferential section of the regular medium 61, and the data area 62 is provided therebetween, as illustrated in FIG. 15. One example of the information recorded in the medium information area 63 concerning the regular medium 61 is medium identification information, which indicates itself as a regular medium.
FIG. 14 illustrates an enlarged view of section b shown in FIG. 15. The data area 62 and the medium information area 63 are recorded with respective information by use of pre-pits that are formed by concave portions and/or protrusions and that are longer than a resolution limit of an optical system in a reproducing device.
The following description explains the conceivable super-resolution medium 71. FIG. 17 illustrates an outer appearance of the super-resolution medium 71. The super-resolution medium 71 has a data area 72 and a medium information area 73. The data area 72 is recorded with information to be used by a user. The medium information area 73 is recorded with information concerning the super-resolution medium 71. The data area 72 and the medium information area 73 are arranged so that the medium information area 73 is provided at an innermost circumferential section and at an outermost circumferential section of the super-resolution medium 71, and the data area 72 is provided therebetween. One example of the information recorded in the medium information area 73 concerning the super-resolution medium 71 is medium identification information, which indicates itself as a super-resolution medium.
FIG. 18 illustrates a cross sectional view of the super-resolution medium 71. The super-resolution medium 71 includes a substrate 74, and a reflection, layer 75 and a super-resolution reproduction layer 76 layered on the substrate 74 in this order by sputtering. The super-resolution reproduction layer 76 causes a super-resolution phenomenon to occur. Furthermore, a transparent cover layer 77 is provided on the super-resolution reproduction layer 76.
FIG. 16 illustrates an enlarged view of section c shown in FIG. 17. The data area 72 and the medium information area 73 is recorded with respective information by use of pre-pits that are formed by concave portions and/or protrusions and that are shorter than a resolution limit of an optical system in a reproducing device. As such, in the super-resolution medium 71, information is recorded by a combination of pre-pits having a length shorter than the pre-pits of the regular medium 61. This allows high density recording of information, thereby allowing recording of more information.
With the super-resolution medium described in Patent Documents 1 and 2, the greater the power of the reproduction laser, the steeper the temperature distribution in the laser spot, thereby resulting in an enhancement in super-resolution effect. Even with the super-resolution medium described in Patent Document 3, although the cause is not understood, the super-resolution effect is enhanced, with greater reproduction laser power, as similar to the super-resolution mediums described in Patent Documents 1 and 2. Therefore, reproduction laser power optimum for reproducing the super-resolution medium 71 is greater than reproduction laser power optimum for reproducing the regular medium 61.
As described above, recording density of the super-resolution medium 71 is higher than recording density of the regular medium 61. Therefore, a reproduction clock width optimum for reproducing the super-resolution medium 71 is narrower than a reproduction clock width optimum for the regular medium 61.
The following considers a reproducing device having compatibility with both the regular medium 61 and the super-resolution medium 71, the reproducing device being capable of reproducing both mediums.
In order to reproduce the regular medium 61 and the super-resolution medium 71 at their most optimum state by the reproducing device, it is required to switch the reproduction laser power and a reproduction clock (this is because the reproduction laser power and the reproduction clock of each of the regular medium 61 and the super-resolution medium 71 are different, as described above). In order to do so, it is required to identify whether a medium is the regular medium 61 or the super-resolution medium 71. Such identification requires to reproduce (i) the medium identification information recorded in the medium information area 63 and (ii) the medium identification information recorded in the medium information area 73.
The identification of the medium is desirably carried out by using the reproduction laser power for the regular medium 61, in order to prevent increase in electricity consumption and breakage of the regular medium 61 caused by the high reproduction laser power for the super-resolution medium 71. However, in such a case, the medium identification information in the super-resolution medium 71 may not be reproducible, thereby causing a problem that no identification can be carried out.
It is an option that in the case the identification could not be carried out by using the reproduction laser power for the regular medium 61, the optical information recording medium to be identified is determined as the super-resolution medium 71, and accordingly the reproduction laser power and the reproduction clock are switched to the appropriate ones. However, excessive time is required since the switching of the reproduction laser power is carried out after the determination is made. As a result, a long time is required to start-up the reproducing device. Furthermore, even in a case where reproduction could not be carried out due to some kind of cause even if the optical information recording medium to be identified is the regular medium 61, the reproducing device mistakenly determines that the regular medium 61 is the super-resolution medium 71. In response to this determination, the reproduction laser power is switched from the reproduction laser power for the regular medium to the reproduction laser power for the super-resolution medium 71. As a result, this may cause a breakage problem of the regular medium 61. The start-up time of the reproducing device denotes a time period required to carry out a series of processes from arranging the optical information recording medium in the reproducing device to reproducing a data area of the optical information recording medium.
For simplification, the above description is described by limiting items which the medium identification information can identify to a regular medium and a super-resolution medium, however it is not limited to this. For example, conventionally, the medium identification information is used for identifying a type of a medium (e.g. CD, DVD, BD) and a form of a medium (e.g. R, RE, ROM).